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Full text of "Mathematical And Physical Papers - Iii"

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360          ON THE  CHANGE  OF  RKFUAXUlBILITY   OK  UOHT.
true dispersion; nor do the theoretical views which I entertain of the cause of the latter lead me to regard it as at all impossible that a beam polarized in the plane of reflexion, and having the same refrangibility as the incident light, may be a necessary accompaniment of true dispersion. However, observation, I think, points in a contrary direction; for although more or less of false dispersion is almost always exhibited along with true dispersion, the quantity of the former seems to have no relation to the quantity of the latter, but does seem to have relation to the greater or less degree of clearness which we should be disposed to attribute to the fluid.
180.    The phenomenon of false internal dispersion seems to admit of being applied as a chemical test to determine whether or not precipitation takes place.  Thus, if a little tincture of turmeric be greatly diluted with alcohol, and then water be added, a yellow fluid is obtained which appears to be perfectly clear, exhibiting no sensible opalescenco; but the occurrence of a copious false dispersion when the fluid  is  examined  by  sunlight,  reveals   at   once,   the existence of suspended particles, though they are too minute to be seen individually, or even to give a discontinuous appearance to the falsely dispersed beam.    Although such a precipitation could not, I suppose, be used as a means of mechanical separation, it might still be useful as pointing out  the possibility of an actual separation under different circumstances as to strength  of solution, &c.
181.    One of the best instances of false dispersion that I have met with, best, that is, in forming a most excellent imitation of true dispersion, occurred in the ease of a specimen of plate-glass which was made, as 1 was informed, with a quantity of alkali hardy sufficient.    This glass,  which was very slightly  yellowish   brown, when viewed edgeways by transmitted light, had a bluish appearance when viewed properly, strongly resembling that of a decoction of the  bark  of the  horse-chestnut, diluted  with   water till   t.ho dispersed light is no longer concentrated in the neighbourhood of the surface.    But when the glass was examined by sunlight, the polarization of the dispersed beam, and the identity of its refra,ngi~ bility with that of the incident light, showed that this was merely an instance of false dispersion.    Another very good  example of